1. Field of the Invention
The present invention relates to a hydrogen storing alloy electrode which can electrochemically absorb and desorb hydrogen in a reversible manner.
2. Description of the Related Art
Storage batteries, which are widely used as a variety of power sources, have two general classifications: lead storage batteries and alkaline storage batteries. Compared with lead storage batteries, alkaline storage batteries have higher reliability and can be made smaller in size, so that they are used in various kinds of portable electric devices. On the other hand, large-sized alkaline storage batteries are widely used for industrial equipment. The positive electrodes of alkaline storage batteries have previously been of a pocket type, but at present, those of a sintered type are generally used. This change in the configuration of the positive electrodes has improved the characteristics of the alkaline storage batteries. Furthermore, it has become possible to hermetically seal the batteries, so that the alkaline storage batteries have come to find wider application.
Some alkaline storage batteries use, for example, air or silver oxide for their positive electrodes. But in most cases, the positive electrodes are made of nickel.
As the negative electrodes of alkaline storage batteries, cadmium electrodes are now generally used. Other materials for the negative electrodes include zinc, iron, hydrogen, and the like.
The cadmium electrodes, however, have a small theoretical energy capacity. Zinc electrodes also have disadvantages in that they are susceptible to deformation and that dandrites tend to be formed on the electrodes.
In order to attain a larger theoretical energy capacity, an electrode made of a hydrogen storing alloy or a hydride thereof has been developed to be used in a nickel-hydrogen storage battery. Many methods for the production of such a nickel-hydrogen storage battery have been proposed.
A hydrogen storing alloy can absorb and desorb hydrogen through a reversible electrochemical reaction. When the negative electrode of an alkaline storage battery is made of this hydrogen storing alloy, it attains a larger theoretical energy capacity, as compared with the cadmium electrodes. Furthermore, unlike the zinc electrodes, when using the hydrogen storing alloy electrode, neither deformation of the electrode nor dendrite-growth on the electrode occurs. Therefore, it is expected that hydrogen storing alloys will provide electrodes having a large theoretical energy capacity and a longer lifetime with no possibility of causing pollution.
Examples of well known hydrogen storing alloys to be used for electrodes include multi-element alloys such as Ti--Ni type alloys, La(or Mm)--Ni type alloys (wherein Mm is a misch metal), and the like.
The Ti--Ni type alloys are of an AB type (wherein A is an element with a strong affinity for hydrogen such as La, Zr and Ti, and B is a transition element such as Ni, Mn and Cr). When such a Ti--Ni type alloy is used for the negative electrode of an alkaline storage battery, the electrode exhibits relatively large discharging capacities in the early charging and discharging cycles. With the repetition of charging and discharging, however, it becomes difficult to keep the discharging capacity from decreasing.
The multi-element alloys of the La(or Mm)--Ni type are known as AB.sub.5 type multi-element alloys. Many attempts have recently been made to develop alloys of this type because they have been considered as suitable materials for electrodes. Moreover, an electrode using this type of alloy has problems of a relatively small discharging capacity and a short lifetime. Furthermore, materials for this type of alloys are expensive.
Another hydrogen storing alloy is a Laves phase alloy of an AB.sub.2 type. The Laves phase alloys have a large capacity for absorbing hydrogen, and electrodes using these alloys have a large energy capacity and a long lifetime. As examples of the Laves phase alloys, there are a Zr.sub..alpha. V.sub..beta. Ni.sub..gamma. M.sub..delta. type alloy (Japanese Laid-Open Patent Publication No. 64-60961), an A.sub.x B.sub.y Ni.sub.z type alloy (wherein A is Zr alone or including 30 atomic % of Ti and Hf, and B is at least one element selected from the group consisting of transition metal elements such as Nb, Cr, Mo, Mn, Fe, Co, Ca, Al, La and Ce) (Japanese Laid-open Patent Publication No. 1-102855), and the like. Further, alloys having improved discharging characteristics in the early charging and discharging cycles (hereinafter referred to as the "early discharging characteristics") are known (Japanese Patent Application Nos. 3-66354, 3-66355, 3-66358 and 3-66359).
Electrodes using the above-mention conventional Laves phase alloys of the AB.sub.2 type have a long lifetime and a large discharging capacity, as compared with electrodes using the multi-element alloys of the AB type such as the Ti--Ni type alloys and the AB.sub.5 type such as the La(or Mn)--Ni type alloys. However, the conventional Laves phase alloy electrodes have poor early discharging characteristics.
The present inventors have improved the early discharging characteristics, while maintaining a large discharging capacity, by using a Zr--Mn--V--M--Ni type alloy (wherein M is at least one element selected from the group consisting of Fe and Co) (Japanese Patent Application No. 4-70704). However, in an enclosed alkaline battery with an electrode using this type of alloy, a large amount of the alloy composition elutes into the alkaline electrolytic solution at high temperatures and the eluted metal element will be precipitated as a conductive oxide. As a result, a short circuit is caused and the battery voltage is rapidly decreased when the battery is kept at 65.degree. C.
Thus, there has been a great demand for a hydrogen storing alloy electrode having a large discharging capacity, excellent early discharging characteristics, and a long lifetime at a high temperature, by further improving the characteristics of the hydrogen storing alloys.